With increasing demands for communication, a wavelength division multiplexing (WDM) transmission technique that uses a digital coherent optical transmission scheme and achieves a transmission rate of, for example, 100 Gps for light with a single wavelength is being actively researched and developed. For the digital coherent optical transmission scheme, multilevel modulation such as dual polarization (DP)-quadrature phase shift keying (QPSK) using a polarization division multiplexing scheme is used in order to achieve a high transmission rate.
In addition, a receiving device that uses the digital coherent optical transmission scheme not only includes an amplifier provided with an erbium doped fiber (EDF), but also amplifies a polarized multiplexed optical signal by a Raman amplifier in order to secure an optical level sufficient for a reception process. It is preferable that the power of polarized light components of the polarized multiplexed optical signal be uniform in order to improve reception performance of the receiving device. The power of the polarized light components is not uniform due to polarization dependent losses (PDLs) of an optical fiber of a transmission path and an optical part existing on the transmission path and a polarization dependent gain (PDG) occurred due to Raman amplification.
The polarization dependent gain occurs based on a relationship between the polarized multiplexed optical signal and a polarization state of excitation light to be used for Raman amplification executed on the polarized multiplexed optical signal. For example, if the excitation light is a linearly-polarized wave, and one of polarized light components of the polarized multiplexed optical signal is parallel to a polarized component of the excitation light, the polarized light component causes an increase in an amplification gain, but the other polarized light component is not parallel to the polarized component of the excitation light and causes a reduction in the amplification gain.
Thus, in order to reduce the polarization dependent gain, a depolarizer configured to reduce a degree of the polarization (DOP) of the excitation light is used. This technique is disclosed in, for example, Japanese Laid-open Patent Publication No. 2003-185852. The degree of the polarization is a value in a range of 0 to 1. If the polarization state is completely destabilized (randomized), the degree of the polarization represents 0. If the polarization state is completely maintained at a certain level, the degree of the polarization represents 1. Specifically, the excitation light is depolarized by passing through the depolarizer.
The depolarizer has a polarization maintaining fiber of which the length (of, for example, several tens of meters to several hundreds of meters) is optimally adjusted based on characteristics of a light source of the excitation light. Thus, the degree of the polarization of the excitation light is not sufficiently reduced (for example, a range of 0.1 to 0.15) depending on the polarization dependent loss of the transmission path and the quality of communication is reduced.